TINY PATCH: ActiveRecord::Base.subclasses is not publicly accessible

"subclasses" class method is not accessible on ActiveRecord::Base
class, but it is on other classes like ActionController::Base.

Knowing the subclasses of AR::Base is pretty useful for every day
usage. Currently, I have to resort to the "send hack" to access this
information (not considering scanning the ObjectSpace an option).

ActiveRecord::Base.subclasses returns a new Array each time it is
called, so should not be possible to affect AR sane behavior by
manipulating the array returned.

Patch is attached to the ticket: